It's not something you get a choice in. If you have anxiety, like an actual anxiety disorder, that's not some sort of weakness or failure to properly "organise" your life. It can happen for any number of reasons which can be out of your control (hormones, PTSD...) It's not realistic to expect someone who is really anxious to just calm down any more than it is to shout at someone with a broken leg to get up and run.
There are things you can do to try and calm down when anxious (like being aware of your surroundings, taking note of something you can see, something you an hear, something you can smell, to ground yourself in the present moment, as well as things like meditation, breathing techniques, regular exercise). If that's not helping you can see a doctor for advice and maybe look into therapy or medication.